As you can see in the thread title, it clearly states "instead of heroicons".
Don't know where you got that idea from.
I will make a photoshop later.
However you are right when you say it would be difficult to see on low resolutions. I will look if this is the case.
Btw with only 7 pixels you would be able to already have a sign for every hero in the game.
The advantage over just hero colors is that if you learn the signs, it gets instinctive. Your brain goes straight from the sign to the hero, while with Xes and Os, your brain has to take the detour over the hero color, so it's Symbol > herocolor > hero, which is not instinctive.

It's hard to argue something that's purely aesthetic when there are no examples.

Okay, finally I experimented in Photoshop and came to the conclusion that the size of the minimap icons is too small for that :X
I thought of this as a compromise between Hero Icons and Only Colors on the map, but it looks like it is not realizable with this setup.
I'm sorry that I wasted your time.

However, I was able to create a mod that changes the Crosses Do I have an advantage now?^^